1. Field of the Invention
This invention relates to a method for preventing discoloration from occurring at the area around indentation in the welded zone of a sheet metal under the influence of heat during spot welding.
2. Description of the Prior Art
As is well known, spot welding is a type of resistance welding method in which two metal sheets to be joined are held and strongly pressed between the rod-shaped pure copper or copper alloy electrodes and a high electric current is passed therethrough for a short period of time to form a so-called nugget of a small disc shape at the contact area between said two metal sheets. The nugget is invisible from the outside, and only a slight indentation formed as a result of strong holding by the electrodes can be seen on the surface on each side of the metal sheets welded.
Thanks to its many advantageous features such as high working rate, high welding efficiency and applicability to welding of light alloys, spot welding is widely utilized in the manufacture of various industrial products such as aircraft, automobiles, railway vehicles and other similar or smaller-sized products such as household goods. However, in certain types of products which are put to use without their surfaces coated with paint, such as stainless steel, common steel, aluminum, aluminum alloy, copper or copper alloy products, and etc., spot welding involves the serious problem of discoloration that occurs at the area around indentation in the spot welded zone. Such discoloration not only impairs the external appearance of the product but also necessitates a great deal of labor and time for its elimination which requires an extra finish treatment.
The cause of occurrence of such discoloration on the spot welded metal surface is explained as follows: as electric current is applied to the electrodes holding the metal sheets to be joined, the weld zone is heated to a high temperature and the surrounding area, exposed to the ambient atmospheric air, is oxidized by the action of oxygen in the atmospheric air, thereby forming a ring-shaped yellow- or brown-colored (discolored) region.
The central part of the welded zone to which the ends of the electrodes are pressed remains almost free of discoloration, because this area is substantially kept out of contact with atmospheric air and also the inside of the electrodes are cooled by cooling water which circulates therein so that the electrodes themselves will not be overheated and softened.
As countermeasures to such phenomenon of discoloration, there are known roughly the following two types of methods: the discolored surface area is decolored by means of electrolytic polishing or pickling; and, as schematically illustrated in FIG. 1, the surface areas 02a near the welded zone of each sheet metal 02 being joined, against which the ends of the rod-shaped electrodes 01 abut, are showered with cooling water 03 throughout the period of electric current application to cool the heated areas to thereby prevent the discoloration from occurring in these areas. Either of these methods has been suitably employed.
Of these conventional practices, the former decoloring method is an operation for eliminating the discoloration after it has once been allowed to occur. Such operation is purely an extra step, and the labor and time used therefor may be said to be redundant. On the other hand, the latter method, i.e., a discoloration preventing method, is incapable of consistently affording the desired cooling effect as the water supplied to the area around the indentation on the welded metal surface 02a readily boils. Also, use of a large volume of cooling water for, giving the desired cooling effect leads to not only a undesirable working environment but also an adverse effect on the quality of spot weld itself. It is thus very difficult with this method to consistently effectuate the necessary and sufficient cooling of the heated weld zone and, therefore, one could not expect an always perfect discoloration preventing effect by this method.